1. Field of the Invention
The invention relates generally to the design and use of cutters for the cutting arms and blades of underreamers, mills and other downhole tools.
2. Description of the Related Art
Rotary cutting mills and mandrel cutters are devices that are incorporated into a drill string and used to cut laterally through metallic tubular members, such as casing on the sides of a wellbore, liners, tubing, pipe or mandrels. Mandrel cutters are used to create a separation in metallic tubular members. Cutting mills are tools that are used in a sidetracking operation to cut a window through surrounding casing and allow drilling of a deviated drill hole. On conventional tools of this type, numerous small individual cutters are attached to multiple arms or blades that are rotated about a hub. Most conventional cutters present a circular cutting face. Other conventional cutter shapes include square, star-shaped, and trapezoidal, although these are less common. However, the use of circular cutters has some inherent drawbacks when used to cut through metallic tubular members. First, there is a small amount of bond area between the cutter and the arm or blade upon which the cutter is mounted. The bond area is essentially the area of the circle. During cutting, the cutters may become loose and break off of the cutting arm. Additionally, the geometry of circular cutters results in a significant amount of interstitial space between cutters. This is detrimental, particularly, when the cutter is cutting through metal that is ductile, such as casing containing high amounts of chrome and/or nickel. These materials will enter the interstitial spaces and erode away the cutting arm during cutting.
In the instance of a rotary cutting mill, the presence of large interstitial spaces also presents a significant problem because of the cutting pattern provided by the mill. As the mill is rotated, the cutters are caused to cut repeatedly along particular paths in the material being cut. This repeated pattern of cutting will result in grooves in the cut material and undesirably force the uncut portions of the material lying between the grooves into the interstitial spaces. To prevent this from happening, half-circular cutters have been used on alternate blades to provide an offset. However, these half-cutters have little bonding area and are prone to breaking off.
Mandrel cutters have at least one cutting knife that is rotated to cut circumferentially through a surrounding metallic tubular member. Mandrel cutters are problematic because they require the use of cutting portions that are very small and narrow in order to effectively cut through the mandrel. The limitation on the size of the cutting portion exacerbates the bonding area problem described above.
The present invention addresses the problems of the prior art.